This invention relates to a method and apparatus for charging a battery, such as an industrial battery, and more particularly, to such a method and apparatus whereby the battery is charged quickly and accurately to the proper charge level. The apparatus described herein is referred to as a "smart" battery charger.
Typical industrial-type batteries, sometimes referred to as "traction" batteries, are used commonly to drive motor vehicles in factories, warehouses, and the like. In such industrial applications, it is necessary that batteries whose charges have been depleted be quickly re-charged for subsequent re-use. Such batteries exhibit high charge capacities, on the order of several hundred ampere-hours (A.H.), and usually are acid-type batteries, such as the conventional lead-acid battery. Desirably, such industrial batteries should be charged by apparatus that is relatively simple to operate.
A typical lead-acid battery, when charged above a predetermined level, exhibits a "gassing" condition, wherein the electrolyte is sufficiently agitated so as to emit gas. In many commercially available battery chargers, energy is supplied to charge the battery until the gassing condition is attained, and charging then continues for a predetermined time after the onset of this condition. Thereafter, an "equalizing" charge is supplied to the battery to balance inherent losses therein. To avoid damage to the battery, when the gassing level is attained, as when a substantially discharged battery is charged, the current supplied during the gas period should be relatively low. Some chargers supply constant charging currents for a pre-set time duration. Depending upon this overall charge time duration, the charging current is established accordingly. For example, a predetermined percentage of the rated battery capacity (A.H.) is supplied for the charge duration; and this percentage is inversely related to the overall charging time. Hence, if the battery is to be charged over a substantially long duration, a low charge current level is used. Conversely, if the battery is to be charged over a relatively short duration, a higher charge current level is supplied. This suffers from the disadvantage of supplying either too high a current during the gassing period, thereby resulting in damage to the battery, or too low a current such that the battery is not satisfactorily charged to its proper level.
Other battery chargers employ the so-called constant voltage technique wherein a constant charging voltage is applied across the battery for the pre-set charging duration. However, this technique does not take into account a change in the actual charge capacity of the battery due to repeated charging operations and, thus, becomes less effective as the battery ages.
Another disadvantage of commercially available battery chargers is that many establish a pre-set charging time which, generally, is unrelated to the actual charge level of the battery. The battery then is charged with a substantially constant charging current which is established by the operator as a function of the rated battery capacity. Since, over a period of time, the actual battery capacity may differ substantially from its rated capacity, the charging current might be too high. Also, if the battery exhibits a relatively low charge level, the charging time duration may be insufficient to charge the battery satisfactorily.
A battery charger has been introduced by Christie Electric Corp., Los Angeles, California, which charges the battery by interspersing negative, or discharge, pulses during the charging operation; with the number of negative pulses being a function of battery capacity. The state-of-charge of the battery is measured by, for example, an ammeter, which indicates battery current during the negative pulse duration, and this negative pulse duration is increased as the state-of-charge increases. However, this charger suffers from many of the aforenoted disadvantages, such as operating over a pre-set charging time duration which may be too short or too long. Also, this charger operates with relatively low capacity batteries.
A battery charger introduced by Westinghouse Davenset Rectifiers of England supplies a charging current to the battery and senses when the battery voltage reaches a predetermined level. This level is assumed to be the gas voltage (V.sub.gas) level, whereupon a pre-set timer is triggered to establish the gas period. At the termination of this pre-set gas period, normal charging is terminated; and the battery then is supplied with an equalizing charge followed by a "hold ready" charge which replaces open circuit losses. While this charger includes various features, such as pre-gas charge protection, avoiding the establishment of the gas period if a fully charged or slightly discharged battery is used, charging nevertheless occurs during a pre-set time interval. There is little, if any, correlation between the actual state-of-charge of the battery and the charging duration. Hence, this charger may, undesirably, either over-charge or under-charge a battery connected thereto.
In the battery charger manufactured by Oldham/Harmer & Simmons, of England, a charging current is supplied to the battery, and this current is measured for a brief period of time when the battery voltage reaches its gas voltage level V.sub.gas. This sequencing between charging and measuring cycles continues until the amount of measured current in two successive measuring cycles is equal. At that point, the battery is considered fully charged. Thereafter, a low level of equalizing charge is supplied. This charger, however, operates substantially independently of the actual condition of the battery. Although the charging cycle is terminated when two successive current measurements are equal, this does not necessarily mean that the battery has been fully charged. Furthermore, the charging current levels are dependent upon the rated battery capacity; which may differ substantially from the actual capacity thereof. Another disadvantage of this charger, which is common with the aforementioned chargers, is that actual battery operating and fault conditions are not sensed automatically or indicated to an operator. Hence, faulty or defective batteries may be charged, with a resultant waste in energy and time. Furthermore, a faulty battery, which may be easily repaired, if not indicated, may be supplied with a charging current that results in permanent damage.
It also has been proposed by the prior art to supply a charging current to a battery, and to interrupt the charging current periodically to measure the battery voltage. This measure of battery voltage is used to indicate whether the battery has reached its gassing level, and the charging current magnitude is reduced when this gassing level is attained. Although the battery voltage is "tested" periodically, such tests are not used to indicate the condition of the battery, nor are the test results used to indicate when the battery has been satisfactorily charged.
The prior art also has proposed a battery test arrangement wherein a current ramp is supplied, resulting in a change in the battery voltage. The voltage-current characteristic for each cell, which is a function of the supplied current and measured voltage, is determined by a computer; and an average voltage-current characteristic is derived from all of the cells. Then, the voltage-current characteristic of each cell is compared to the average voltage-current characteristic, and the battery is rejected if the characteristic of any one cell differs significantly from the average characteristic. However, this arrangement requires direct access to each cell of the battery. In many industrial batteries, such access is difficult, if not impossible. Furthermore, since the battery is "tested" by deriving an average voltage-current characteristic therefrom, a battery which is severely defective will not be detected. Still further, although this prior art proposal describes a battery test technique, that technique is not used to control a battery charging operation.
Yet another prior art battery test technique proposes that a voltage-current curve derived from the tested battery be plotted, and that the slope of this curve be compared to an "average" curve which represents average slopes for different battery charge levels. Then, the actual charge level of the battery under test is determined by noting the charge level on this "average" curve corresponding to the measured slope of the test voltage-current curve. Here too, however, the battery test technique is not used to control a charging operation. Furthermore, this test technique, although helpful in obtaining a measure of the state-of-charge of the battery, nevertheless does not provide indications of various fault conditions which may exist.
Another problem associated with many battery chargers is that the charging current supplied thereby should be matched to the battery which is charged. Typically, it is necessary that the capacity of the battery as well as the number of cells included therein be known in advance. Furthermore, many of these chargers will attempt to charge a defective battery notwithstanding a serious fault condition which may be present. This can result in damage to the battery as well as produce a hazardous condition.
Therefore, there has been a need for a battery charger apparatus which operates substantially automatically, requiring little, if any, advance information concerning the battery capacity, number of cells, and the like. There also has been a need for a battery charger of relatively small size and light weight. Such a charger, desirably, should be capable of testing the condition of the battery and indicate, or display, various fault conditions. Such a charger also should be capable, advantageously, of controlling the charging operation as a function of the instantaneous condition of the battery, and to terminate the charging operation when the battery is fully charged, regardless of the time duration required to attain such a fully-charged state.